1. Field of the Invention
The invention relates to an alkali-soluble resin and a photosensitive resin composition for a color filter in a liquid crystal display device, and a color filter and liquid crystal display device formed by the aforementioned photosensitive resin composition. More particularly, the invention provides an alkali-soluble resin and a photosensitive resin composition for a color filter having good change rate of film thickness in thermal solvent and linearity of pattern with high finesse.
2. Description of the Related Art
Because a liquid crystal display device has properties of lightweight, thin shape, low power consumption and so on, it is used in various applications such as a notebook, a personal digital assistance, a digital camera and a desktop computer screen. The aforementioned liquid crystal display device needs a color filter having high color reproduction range.
The color filter is obtained by forming three different colors, red (R), green (G) and blue (B), in strips or mosaics on a surface of a support such as a glass, a plastic sheet or the like with a black matrix (BM).
In variety manufacturing methods of the conventional color filter, a well-known method is to use a negative-type photosensitive color composition. The negative-type photosensitive color composition includes a pigment, an acrylic resin, a photopolymerization initiator, a reactive polymer or the like. Curing of the negative-type photosensitive color composition is carried out by the followings: first, producing a free radical by irradiation with ultraviolet light to decompose or activate the photopolymerization initiator, and carrying out a free radical polymerization of a free-radical activated ethylenically unsaturated compound. When the negative-type photosensitive color composition is used for manufacturing the color filter, the negative-type photosensitive color composition is usually coated on the substrate and developed after irradiated with ultraviolet rays via a mask to obtain a pattern. Then, the aforementioned pattern is heated to calcine for fixed adhesion on the substrate to form a pixel pattern. The process is repeated according to the necessary colors to obtain a pattern of color coating film. However, because of the process is repeated, the end of the pixels of BM and RGB will have greater segment deviation and the segment deviation will cause color displaying unevenly. In order to inhibit the segment deviation, a transparent resin layer (protective film) is used in this field to carry out a planarization treatment for the color filter.
The protective film must to have properties of protecting the RGB color layer, heat resistance for liquid crystal filling, hardness for pressure resistance and the like. At the same time, in order to meet the requirement of hardness, a photosensitive curing resin composition with high crosslinking density is developed (Japanese Patent Publication No. 05-78483). However, the photosensitive curing resin composition will shrink and generate residual stress, and it is easy to have a defect of poor linearity of pattern with high finesse.
Japanese Patent Publication No. 10-133372 discloses a protective film composition containing an epoxy compound. Although it can improve the problem of linearity of pattern with high finesse, it still has a defect of poor change rate of film thickness in thermal solvent.
In another aspect, in recent years, because the personal digital assistant and digital camera have tended to miniaturization and lightweight, the color filter needs further thinning and high color saturation; thus, it is necessary to increase the concentration of a colorant in the color composition. However, if the concentration of the colorant is raised, the resin amount of the photosensitive resin composition relatively becomes less. As the amount of the pixel surface resin contributing to adhesion of the protective film becoming less, the adhesion of pixel and the protective film reduces, and the protective film is easy to peel off from the interface with the pixel, and it further causes poor linearity of pattern with high finesse.
To improve the aforementioned problems, a polyfunctional photocuring monomer having polymerizable unsaturated bonds, an alkali-soluble resin and a photopolymerization initiator are used in the aforementioned photosensitive resin composition in the field. For example, Japanese Patent Publication Nos. 61-213213 and 01-152449 disclose uses of a color filter material. A copolymer polymerized by (meth) acrylic acid having a carboxyl group or by (meth) acrylate, maleic anhydride and other polymerizable monomers as an alkali-soluble resin is disclosed. However, the copolymer is a random copolymer, and it is difficult to obtain better linearity of pattern with high finesse and change rate of film thickness in thermal solvent under developing operation because distribution difference of alkali dissolving rate occurs in the light irradiated portion between the non-irradiated portion.
Therefore, improving the change rate of film thickness in thermal solvent and linearity of pattern with high finesse simultaneously in order to meet the modern requirements is a target remained to be achieved.